Back in the days before computer magic could make anyone sound like a super-star singer, you needed to get an actual singer to cover for actors who couldn't sing well enough. Maria's singing parts in the film were performed by Marni Nixon, a legendary performer known for providing the singing voice for a number of leading ladies.

Besides Wood, she also dubbed in two other blockbusters: Deborah Kerr's voice in The King and I and Audrey Hepburn's voice in My Fair Lady. Wood didn't realize all her singing would be dubbed until after the filming was over, and she wasn't happy about it. (Source)

Plenty of people have been unhappy about the lack of minority actors in minority roles, Natalie Wood and George Chakiris being famous examples of that. West Side Story actually helped break one barrier for minority actors: Rita Moreno became the first Latina actress to ever win an Oscar. Chakiris, who won an Oscar for playing Bernardo, has Greek ancestry. (Source)

It you pay close attention, the first words of dialogue in West Side Story are "beat it." Fans of the Michael Jackson should recognize those words: they're the title to one of his most famous songs of all time, from his signature 1982 album Thriller. The King of Pop was supposedly a big fan of West Side Story, and a rumor started that the song is basically his homage to all things Jets and Sharks. Great story, right? Unfortunately, the music video's director shot it down. (Source)

Robert Wise's first choice to play Tony was Elvis, but his manager turned the offer down. Elvis had already played street kids in Jailhouse Rock and King Creole, and they wanted him to stick to some nice-guy roles. (Source)

Leonard Bernstein hinted that the famous three-note Jets theme (you can hear it at the 5:07 mark here), which becomes a musical motif throughout the score, was influenced, maybe unconsciously, by the shofar blast—the shofar being the ram's-horn instrument blown in a synagogue on certain Jewish holidays. In Biblical times, the shofar was used to announce holidays, and also to signal the beginning of a war. It's a kind of wake-up call. Check it out and see what you think. (Source)